1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image using an electrophotographic system, particularly to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction peripheral including plural functions thereof.
2. Description of the Related Art
Conventionally, in the image forming apparatus which forms a color image, there is a system in which toner images are formed using four color toners of yellow, magenta, cyan, and black and fixed while superposed. Generally, in some image forming apparatuses provided with the electrophotographic system, a two-component developer including a non-magnetic toner particle (toner) and a magnetic carrier particle (magnetic carrier) is used as a developer. Particularly, in the color image forming apparatus, the two-component developer is widely used for the reason that a shade is good because the magnetic material is not included in the toner.
In the color image forming apparatus, it is necessary to stabilize a color of an output. Therefore, for example, Japanese Patent Laid-Open Nos. 09-015963 and 05-289464 propose an attempt to stabilize the color of the output by stabilizing a density of each color.
In Japanese Patent Laid-Open No. 09-015963, a detector is used to detect the density of a test reference image (patch image) formed on an image bearing member. Another detector is used to detect a developer toner concentration in a developing container. A toner replenishing control system is switched based on detection results of the patch image density and developer toner concentration.
A development characteristic changes when a toner charge amount (triboluminence) changes by alteration of the magnetic carrier in the developer or an environmental fluctuation. Accordingly, a toner adhesion amount (that is, image density) of the patch image on the image bearing member indicates the development characteristic based on the change in toner charge amount. In order to guarantee the change in image density, developer toner concentration in the developing container is changed according to the change in toner adhesion amount, and control is performed such that the toner adhesion amount is kept constant.
However, in the case that toner replenishment decreases to significantly decrease the developer toner concentration as a result of the toner adhesion amount constant control, a coating amount decreases on a developing sleeve to lead to image degradation due to magnetic carrier adhesion. In the case that the toner replenishment increases to significantly increase the developer toner concentration, the developer overflows or the toner is transferred to a sheet white background part which should not originally be printed, which results in what is called an “image fog” in which the white background part gets dirty.
In Japanese Patent Laid-Open No. 09-015963, usually image density constant control is performed in order to guarantee the change in developer characteristic with the patch image density. As described above, in order to suppress runaway of the developer toner concentration in the developing container, the toner replenishing control system is switched in the case that the developer toner concentration in the developing container is greater than or equal to the predetermined range or less than or equal to the predetermined range.
In the technology of Japanese Patent Laid-Open No. 05-289464, using a detector which detects the test reference image (patch image) density formed on the image bearing member and a detector which detects the developer toner concentration in the developing container, a developing contrast potential is changed based on the results of the patch image density and developer toner concentration. At this point, the toner adhesion amount changes because toner charge amount changes by the alteration of the magnetic carrier in the developer or the environmental fluctuation. Therefore, the toner adhesion amount constant control is performed by changing the developer toner concentration in the developing container.
As to the problem in that the image density increases or decreases due to the developer toner concentration constant control, the change in image density is suppressed by increasing or decreasing the developing contrast potential.
In the shade stabilizing technologies of Japanese Patent Laid-Open Nos. 09-015963 and 05-289464, the color of the output is stabilized by stabilizing the color toner concentrations of yellow, magenta, cyan, and black. However, in the technologies, since a countermeasure is individually taken against the yellow, magenta, cyan, and black developing devices, sometimes a person recognizes the change in shade.
That is, when yellow, magenta, cyan, and black differ from one another in a tendency of the change in density, the person may recognize the “change in shade” even if the density of each color fluctuates slightly. This is because how the person feels the “change in shade” in a multiple order color such as secondary colors of red, blue, and green.
Specifically, in Japanese Patent Laid-Open Nos. 09-015963 and 05-289464, in order to suppress the change in toner adhesion amount (patch image density), the toner charge amount is kept constant by changing the developer toner concentration. This is the useful technology as the density stabilizing technology. However, as described above, it is necessary that the developer toner concentration fall within a certain range in order to prevent the overflow of the toner from the developing container, the image fog, and the magnetic carrier adhesion.
When the developer toner concentration exists outside the setting range, a transition is made to the developer toner concentration constant control. After the transition to the developer toner concentration constant control, sometimes the stability of the toner charge amount is lost to generate the change in image density. For example, in the case that the patch image density constant control is performed to cyan while the developer toner concentration constant control is performed to magenta, the person may feel the large change in shade of the image density in blue which is of the secondary color.
During the developer toner concentration constant control, the toner charge amount cannot be controlled because a mixture ratio (a ratio of a non-magnetic toner weight (T) to a total weight (D) of the magnetic carrier and non-magnetic toner, hereinafter referred to as a “T/D ratio”) of the non-magnetic toner and magnetic carrier in the developing device is constantly controlled. On the other hand, during the patch image density constant control, the T/D ratio cannot be controlled because the toner charge amount is constantly controlled (that is, toner adhesion amount is constantly controlled). That is, the toner adhesion amount (that is, image density) varies during the developer toner concentration constant control, and the T/D ratio varies during the patch image density constant control.
Therefore, usually the two kinds of control are simultaneously performed with respect to the patch image density constant control in which the toner adhesion amounts of two colors vary slightly. For example, developer concentration constant control is performed to magenta while the patch image density constant control is performed to cyan. In this case, in forming image in blue which is of the secondary color, the variation in image density in magenta leads to the fluctuation in image density (color difference) in all the blue colors. Therefore, in the case that the image is viewed as the blue color, the person feels the change in shade by a density difference of magenta.
The shade will be described in detail. Generally the color is expressed by a color space such as L*a*b* and L*C*h° displayed in a polar coordinate in an a*b* plane. At this point, L* expresses lightness, C* expresses color saturation, and h° expresses a hue. It is said that the person easily recognizes the “change in shade” in the case that the hue h° changes.
The inventors made a simple experiment in order to verify a correspondence between the actual appearance of the secondary color and the hue h°. Half-tone images of yellow (Y), magenta (M), and cyan (C) were output with a full-color copying machine iRC3380 (manufactured by Canon Incorporated). The half-tone level was set to 64 level, 80 level, and 96 level in 0 to 255 levels.
A result in FIG. 5A was obtained when reflection densities of the samples were measured with a spectrophotometer 528JP (manufactured by X-Rite Incorporated).
Then nine kids of the red, blue, and green half-tone images in which two of the single half-tone colors were selected were output by combining the half-tone levels of three stages (64 level, 80 level, and 96 level) of each single color.
Based on the sample in which the single colors has the 80 and 80 levels, Δh° (a difference between h° of the reference sample and h° or a target sample) of the eight kinds of samples were measured with a spectrophotometer 528JP (manufactured by X-Rite Incorporated). FIGS. 5B to 5D illustrate the results.
In FIGS. 5B to 5D, a numerical value “ . . . *” at the 80 and 80 levels means a measurement error, and the numerical value “ . . . *” is originally zero.
In any secondary color, an absolute value of Δh° decreases in upper left and lower right directions in FIGS. 5B to 5D with respect to the sample at 80 and 80 levels, and the absolute value of Δh° increases in lower left and upper right directions, a vertical direction and a horizontal direction. That is, in the case that the density of one of the colors constituting the secondary color decreases (increases), compared with the case that the other color does not change, the change in hue (the absolute value of Δh°) decreases when the density of the other color decreases (increases).
The inventors actually compared the shade by the naked eye while the samples are two-dimensionally arrayed. As a result, as described above, the change in shade was hardly recognized in the upper left and lower right directions in which the absolute value of Δh° decreases, and the change in shade was prominent in other directions. That is, it is found that the value of the hue h° corresponds actually to the actual appearance of the secondary color.
The following items are found from the result. (1) In the case that the changes in density of the two colors constituting the secondary color are oriented in the directions opposite to each other (the lower left and upper right directions in FIGS. 5B to 5D), the change in hue (Δh°) increases, and the person recognizes the large change in shade. (2) The case that the densities of the two colors change in the identical direction (the upper left and lower right directions in FIGS. 5B to 5D) is better than the case that the density of one of the colors constituting the secondary color does not change while only the density of the other color changes (the vertical and horizontal directions in FIGS. 5B to 5D). In the case that the densities of the two colors change in the identical direction, the change in hue (Δh°) decreases, and the person hardly recognizes the change in shade.
That is, in the conventional technology, from the standpoint of the “change in hue in the multiple order color”, since the colors are independently controlled, the person recognizes the change in shade by the change in hue h° even if the density of each color fluctuates slightly.
It is desirable to be able to effectively suppress the change in shade in the image forming apparatus which forms the color image using the plural colored toners.